Communication systems may be modelled in terms of a transmitter 10 and receiver 30, separated by a channel 20, as shown in FIG. 1. The transmitter 10 transforms the data into a signal suitable for transmission over the channel 20. The channel may distort the transmitted signal in some way. The receiver's goal is to remove the effects of the channel distortions from the signal and to transform the signal into an estimate of the original data.
The receiver may include a Channel Estimator. The Channel Estimator may observe a received signal that has been distorted by transmission over the channel, and generate a channel estimate based upon this observation. Channel distortions may include amplitude distortions, frequency offsets, phase offsets, Doppler effects, or distortions resulting from a channel with memory, such as Rayleigh fading, Rician fading, or multipath channels, or additive noise or interference. The receiver may use the channel estimate to remove the effect of the channel and generate an estimate of the data that was transmitted.
In an ideal receiver (a.k.a. a Genie-aided receiver) the channel estimate would be perfect, and the estimate of the transmitted data would be optimal. However, in practice channel estimates may not be perfect, so the estimate of the transmitted data may be sub-optimal. Furthermore, many receivers are designed for operation over a narrow range of channel types. If these receivers are used to receive data transmitted over types of channels they were not designed for, then their channel estimators may be more likely to generate erroneous channel estimates, thereby degrading performance.
The Digital Video Broadcast Handheld (DVB-H) standard, published by the European Telecommunications Standard Institute (ETSI), extends the terrestrial standard (DVB-T). DVB-H aims to specify an efficient means for broadcasting multimedia services to battery-powered handheld terminals. DVB-H is backward compatible with its terrestrial predecessor.
The standards are published as follows:                Digital video broadcasting (DVB); transmission system for handheld terminals (DVB-H), ETSI EN 302 304 V1.1.1 (2004-11), European Telecommunications Standards Institute; and        Digital video broadcasting (DVB); framing structure, channel coding and modulation for digital terrestrial television, ETSI EN 300 744 V1.5.1 (2004-11), European Telecommunications Standards Institute.        
Aims of the DVB-H standard include:                Robust mobile connectivity;        Maximization of coverage;        Low power consumption;        Mitigation of impulse interference; and        User roaming between cells.        
With these aims in mind, the standard includes the components described in Table 1.
TABLE 1DVB-H Specific ComponentsMandatory/ComponentLayerOptionalStated AimTime-SlicingLinkMandatoryReduce average power consumptionand allow seamless handover.DVB-HPHYMandatoryFast service discovery andSignallinghandover.MPE-FECLinkOptionalImprove C/N and Dopplerperformance in mobile channels,and improve tolerance to impulseinterference.4K ModePHYOptionalTrade-off mobility and SFN size.In-depthPHYOptionalImproved robustness for 2K, 4KSymbolmodes.Interleaver
When mobility is present the DVB waveform may suffer from Inter-Carrier Interference (ICI) where, due to mobility-induced Doppler, subcarriers interfere with each other.
To address this problem the DVB-H standard extends DVB-T by addition of a “4K” mode to the 2K and 8K modes. The number here refers to the number of subcarriers in the FFT used to generate the transmitted DVB waveform. For a given bandwidth signal more subcarriers means closer subcarriers and higher vulnerability to Doppler. The 8K mode is particularly vulnerable to Doppler. The 4K mode is seen as a compromise between wider subcarriers, as offered by the 2K mode, and the longer cyclic prefix offered by the 8K mode.
DVB also has a hierarchical mode where a waveform may be demodulated in two ways. One method results in more reliable demodulation at a lower data rate and the other is more difficult to demodulate but results in higher data rates. An ability to demodulate in higher data rate modes offers improved service to end users.
Pilot symbols are inserted in the transmitted waveform in order to enable channel estimation for coherent demodulation and decoding at the receiver. Approximately 1/9th of the subcarriers, in any given Orthogonal Frequency-Division Multiplexing (OFDM) symbol, are used for this purpose in DVB. At the limits of coverage and mobility the ability of a receiver, using these pilots alone, to derive an accurate channel estimate is compromised.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgement or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.